A laser oscillator capable of effectively collecting and removing particulate foreign matter contained in gas laser medium flowing in a circulating path. An electric discharge section arranged in an optical resonator is incorporated into a circulating path of the gas medium including a heat exchanger and a blower. The gas laser medium flows at a high speed in the circulating path. At least one spiral-flow dust collecting mechanism is provided in the circular path downstream of the heat exchanger, and the gas laser medium discharged from the blower is supplied to the electric discharge section through the spiral-flow dust collecting mechanism. The gas laser medium containing the particulate foreign matter spirally flows around an inner pipe in a cylindrical body of the spiral-flow dust collecting mechanism. While spirally flowing in the cylindrical body, the foreign matter having a relative density larger than that of the gas medium is separated from the gas medium by centrifugal force, to introduced into a dust collecting section through a slit to be deposited. Pressure of the gas laser medium flowing in the circulating path may be adjusted in accordance with diameter and/or mass of the particulate foreign matter to be removed.